1. Field of the Invention
The present invention relates to a process for recovering n-hexenes from a mixture of C.sub.6 olefin isomers and a process for producing 1-hexene from a mixture of C.sub.6 olefin isomers.
2. Description of the Prior Art
Compounds having a terminal double bond (hereinafter referred to as "terminal olefins" or ".alpha.-olefins") are very useful industrially as raw materials for heat-resistant polymers, comonomers for the production of polyolefins, starting materials for detergents and so forth. The terminal olefin 1-hexene is especially valuable for many uses such as dimerization to dodecenes which are suitable for making biodegradable detergents, using it as a feed for the OXO reaction to make relatively linear C.sub.7 alcohols, and as a comonomer in making linear low density polyethylene.
A potential source of 1-hexene is a mixture of n-hexenes which contains 1-hexene, cis and trans 2-hexene, and cis and trans 3-hexene. These n-hexenes can be found at appreciable levels (over about 10% by weight) in certain mixtures of C.sub.6 olefin isomers prepared, for example, by the dimerization of propylene. Generally, propylene dimers prepared using acid catalysis do not contain appreciable amounts of n-hexenes, and are, thus, not suitable as a source of n-hexenes. Propylene dimers prepared by transition metal catalysis usually do contain appreciable amounts of n-hexenes, and are useful as a source of n-hexenes. Mixtures of propylene dimers made via nickel or cobalt catalysis are particularly suitable. Depending upon the reaction conditions and the form of the catalyst, the n-hexene content in mixtures of propylene dimers prepared via nickel or cobalt catalysis can vary from zero to 75% by weight. The remaining C.sub.6 olefin isomers in these mixtures of propylene dimers are branched-chain C.sub.6 olefins. These branched-chain C.sub.6 olefins are difficult to separate from the n-hexenes because at least some of them have boiling points very close to those of the n-hexenes.
There are processes known for separating branched-chain C.sub.4 and C.sub.5 olefins from their straight chain isomers by extraction with dilute (about 65%) sulfuric acid. However, this extraction approach is not desirable for mixtures of C.sub.6 olefin isomers because of the poorer extraction equilibrium with C.sub.6 olefins, and the requirement of working with a large volume of corrosive sulfuric acid.
Thus, there exists a need for a process of recovering n-hexenes from a mixture of C.sub.6 olefin isomers containing n-hexenes and branched-chain C.sub.6 olefin isomers.
While, as noted above, mixtures of C.sub.6 olefin isomers can be made which contain appreciable amounts of n-hexenes, the amount of 1-hexene in these mixtures is normally low. For example, thermodynamic equilibration of n-hexenes produces a mixture containing only about 2-4% 1-hexene. While it is possible to separate the 1-hexene from the other n-hexenes in these mixtures, due to the very low levels of 1-hexene, such a procedure would be uneconomical. Thus, there exists a need for a method by which the amount of 1-hexene in these n-hexene mixtures can be substantially increased.
A known method for producing terminal olefins, such as 1-hexene, is to dehydrate a 2-alcohol, i.e., a compound of the formula ##STR1## where R is a hydrocarbyl group. For example, U.S. Pat. No. 3,283,027, issued Nov. 1, 1966 to Lundeen et al., discloses the dehydration of 2-alcohols to terminal olefins (also known as ".alpha.-olefins") using a catalyst which is a thorium, scandium, yttrium or rare earth oxide. While this dehydration reaction can produce an .alpha.-olefin and/or a 2-olefin, the Lundeen et al. product is said to be 90% or more .alpha.-olefin.
U S. Pat. No. 3,600,455, issued Aug. 17, 1971 to Dean, discloses a process for producing the terminal olefin 4-methylpentene-1 by dehydrating 4-methyl pentanol-1 or 4-methyl pentanol-2 by passing it over an alkalized alumina catalyst.
U.S. Pat. No. 4,234,752, issued Nov. 18, 1980 to Wu et al., discloses the dehydration of C.sub.2-20 alcohols in the presence of gamma-alumina (which may be base-treated) employing an inert carrier gas to produce an olefin. The process is said to minimize isomerization which can convert desired products to undesired products. For example, according to Wu et al., 3-methyl-1-butanol can be dehydrated by this process to produce 3-methyl-1-butene having a 97.7 wt. % purity.
U.S. Pat. No. 4,490,567, issued Dec. 25, 1984 to Drake, discloses a process for the selective dehydration of 2-alcohols to .alpha.-olefins using a catalyst which is (1) at least one catalytic metal oxide on a low surface area aluminum oxide-containing support, or (2) a mixture of thorium oxide and cerium oxide on a base-treated aluminum oxide-containing support. Also described is a process for obtaining high purity 4-methyl-1-pentene by the dehydration of 4-methyl-2-pentanol followed by disproportionation with ethylene.
European Patent Specification Publication No. 0150832, published Nov. 2, 1988, discloses a process for preparing .alpha.-olefins by dehydrating 2-alcohols using a high purity (i.e., substantially free of silicon and titanium) zirconium oxide catalyst, and European Patent Specification Publication No. 0222356, published May 20, 1987, discloses the dehydration of 2-alcohols to .alpha.-olefins using a zirconia catalyst which has been treated with an alkaline solution.
Lundeen and Hoozer, "Selective Catalytic Dehydration. Thoria-Catalyzed Dehydration of Alcohols", J. Org, Chem., 32, pp. 3386-3389 (1967) discloses that the thoria-catalyzed dehydration of secondary 2-alcohols is selective for .alpha.-olefins, and that the amount of ketone by-product is low, and Davis, "Catalytic Conversion of Alcohols. 11. Influence of Preparation and Pretreatment on the Selectivity of Zirconia", Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, pp. 181-198 (1979) discloses that a zirconia catalyst is similar to thoria for both the dehydration and .alpha.-olefin selectivity in the conversion of 2-alcohols to olefins.
Other methods of preparing .alpha.-olefins are also known. For example, British Patent Specification No. 1,233,020, published May 26, 1971, discloses a method for making 4-methylpentene-1 by subjecting a mixture of acetone and isobutyraldehyde to conditions under which acetone undergoes condensation both with itself to form diacetone alcohol and with isobutyraldehyde to form the acetone/isobutyraldehyde condensate methyl 2-methyl 3-hydroxy butyl ketone, subjecting the mixed condensates to conditions under which they undergo dehydration to the corresponding olefinically unsaturated ketones, hydrogenating these ketones to saturated alcohols and dehydrating these saturated alcohols over alkalized alumina to form a mixture of 4-methylpentenes-1 and -2 and a mixture of methyl hexenes.
A process for recovering n-hexenes from a mixture of n-hexenes and branched-chain C.sub.6 olefin isomers has now been discovered, as well as a process for producing 1-hexene from a mixture of C.sub.6 olefin isomers.